9
19/02/2002Basi di dati, capitolo 149 The Web: a great opportunity the diffusion of the Web is... the Web (with its browsers) is becoming a standard interface for the final user –the protocol is very simple and public –the interface is uniform –the content is very rich (in breadth and depth) it is becoming a standard interface for accessing many services, with information systems and databases of every type

11
19/02/2002Basi di dati, capitolo 1411 A topical issue a request from our users: –computing facilities should become similar to standard utilities (gas, phone, power, etc.) our usual reply: –computing services have application specific features for which standard services would only be a limited solutions (as it is the case for the other utilities) however –what would a standard offer of services be?

17
19/02/2002Basi di dati, capitolo 1417 Two worlds? Databases can be queried in a flexible way; hypertexts are easy to access, but cannot be queried Web sites are often difficult to explore, use and monitor Database design methods are well known Web sites are difficult to design and maintain

18
19/02/2002Basi di dati, capitolo 1418 Web and DBs: a contradiction? databases are well structured and organized how much structure and organization is there in the Web? it depends, both on the source and on the user there are different degrees of granularity and structure for our data we need to be able to make conversions (from DB to HT and viceversa)

20
19/02/2002Basi di dati, capitolo 1420 Problems with many Web-sites design –information is often poorly organized and difficult to access –it is not even clear which pieces of information are available –the access structure is casual and many dangling references occur –the style of presentation is heterogeneous maintenance –difficulties in updating the content (type and values) –difficulties in changing the initially defined structure –difficulties in changing the presentation details

24
19/02/2002Basi di dati, capitolo 1424 Hypertext data-independence Data what information is offered through the site and what are the conceptual details and the logical organization Hypertext how data is arranged in pages and what navigation links correlate them Presentation the appearance of each piece of information in pages

26
19/02/2002Basi di dati, capitolo 1426 Components and Models dataER and Relational hypertext presentationHTML What is missing is a model for hypertexts!

27
19/02/2002Basi di dati, capitolo 1427 Models for hypertexts in data-intensive Web sites (and often in general) there are (many) pages with a similar (or even the same) structure thirty or forty years ago people realized that in an application it is often the case that there are records with the same structure; files with a rather fixed structure were invented with this purpose the notion of scheme of the database was later introduced as an overall description of the content of a database

47
19/02/2002Basi di dati, capitolo 1447 Navigation Conceptual Model (NCM) Hypertext Conceptual Features Which concepts should be the hypertext nodes Which should be the navigation paths between nodes How nodes should be aggregated to build the hierarchical access structure NCM Constructs Macroentity Directed Relationship Aggregation